Composite body for gas discharge lamp

ABSTRACT

Composite body, a gas discharge lamp having a sodium pressure from 300-600 torr in particular, having an envelope of densely sintered aluminium oxide or of other sodium vapor-resistant materials and a feed-through member of tantalum, niobium or other metals equivalent thereto, the bonding material consisting of a finely dispersed polycrystalline product having a composition in mole % between the following limits: 
     Al 2  O 3  --5-70 
     at least one oxide chosen from Sc 2  O 3  and Ti 2  O x , wherein 
     2&lt;×&lt;4--2-70 
     and at least one oxide chosen from: 
     Y 2  O 3 , Gd 2  O 3 , Tb 2  O 3 , Dy 2  O 3 , Ho 2  O 3 , Er 2  O 3 , Eu 2  O 3 , Pr 2  O 3 , Ce 2  O 3 , Sm 2  O 3 , Tm 2  O 3 , Yb 2  O 3 , Lu 2  O 3 , La 2  O 3  and Nd 2  O 3  --10-80.

BACKGROUND OF THE INVENTION

The invention relates to a composite body formed from two or more parts, at least one of which consists of an oxidic material which may be bonded to one or more parts of a metallic material or a mixture thereof with a metal oxide (cermet), the oxidic material being chosen from densely sintered aluminium oxide (dga), rare earth perowskites of the type ABO₃, wherein A represents La, Ce, Pr, Nd or a mixture thereof and B represents: Al, Sc, Lu or a mixture thereof, garnets of the type A₃ Al₅ O₁₂, wherein A is Mg, Li or Y, or sapphire, and the metallic part being chosen from tantalum, niobium, tungsten, molybdenum, alloys having one of these metals as the main component and alloys having iron, nickel or cobalt as the main component, molybdenum-silicon alloys, titanium boride and titanium nitride, which parts are bonded together in a gas-tight and vacuum-tight manner with a bonding material, the bonding material containing aluminum oxide and at least one oxide of a rare earth metal.

The invention particularly relates to a gas discharge lamp the envelope of which consists of the above-mentioned densely sintered aluminum oxide, which lamp comprises an electrode feed-through of niobium and contains sodium vapour or a metal halide vapour as the gas filling during operation. Densely sintered aluminum oxide is a material which is perfectly suitable for this purpose as it is resistant to the action of sodium vapour and to the action of metal halides, even at temperatures as high as 1200° C.

German Patent Specification 17 71 143 (U.S. Pat. No. 3,926,603) discloses a material for bonding bodies of densely sintered aluminum oxide to each other and to metal bodies, this material having the following composition expressed in % by weight:

CaO--24-50

Al₂ O₃ --35-57.5

MgO--0-12

BaO--0-16

Y₂ O₃ --0-10

B₂ O₃ --0-9

ZrO₂ --0-17,5

Li₂ O--0-3

the sum (B₂ O₃ +ZrO₂ +Li₂ O) being between 3 and 20% by weight.

This vitreous composition is devitrified, preferably after having been applied in the sealing seam, by a thermal treatment to a fine-crystalline product which especially in this state is resistant to attack by sodium vapour at temperature up to approximately 900° C.

Because of the fact that a white light-emitting sodium vapour discharge lamp has been developed in which a sodium pressure of 300-600 torr is used, which is realised by a higher temperature of the coldest spot in the discharge lamp, the need has now arisen for a material which is resistant to sodium and sodium aluminate up to temperatures of approximately 1000° C. and which is suitable for use as a sealing material at a temperature lower than 1750° C., preferably lower than 1650° C.

It has been found that the prior art bonding material on the basis of alkaline earth aluminate is not sufficiently resistant to the corrosive action of sodium atoms and sodium ions. In the above-mentioned, novel discharge lamp the bonding material is exposed at a temperature of 850°-950° C. to sodium vapour having a vapour pressure of 300-600 torr and to liquid sodium in the form of an amalgam. In addition, the bonding material is also exposed to contact with sodium ions originating from a sodium-β-aluminate layer formed during operation. All this results in the formation of unwanted compounds by ion exchange which shorten the life of the bond and consequently of the lamp to an impermissible extent.

SUMMARY OF THE INVENTION

The invention has for its object to provide a class of bonding materials which are capable of resisting the unwanted highly corrosive action of the lamp filling and by means of which the seal in the composite body can be made at a temperature of not more than 1750° C. This makes it possible to produce a high-pressure sodium vapour discharge lamp having an operating life of an acceptable duration.

According to the invention, the composite body is characterized in that the bonding material consists of a finely dispersed polycrystalline product comprising at least three oxidic components and whose composition is within the following limits in mole %:

Al₂ O₃ --5-70

at least one oxide chosen from Sc₂ O₃ and Ti₂ O_(x) wherein

2<x<4--2-70

and at least one oxide chosen from:

Y₂ O₃, Gd₂ O₃, Tb₂ O₃, Dy₂ O₃, HO₂ O₃, Er₂ O₃, Eu₂ O₃, Pr₂ O₃, Ce₂ O₃, Sm₂ O₃, Tm₂ O₃, Yb₂ O₃, Lu₂ O₃, La₂ O₃ and Nd₂ O₃ --10-80.

The bonding material may comprise 0-20 mole % of a crystal growth-inhibiting oxide to make sure that no coarse cristallisation occurs. Such coarse cristallisation sometimes causes cracks at the crystalline boundaries. Suitable examples of such oxides are ZrO₂, B₂ O₃, SiO₂, P₂ O₅ or alkaline earth oxides such as MgO, BaO and SrO or combinations thereof, the choice depending on the basic composition of the bonding material. The effect of some of these admixtures is known per se, for example from the WO-PA 79/00220. The effect of others is new.

Preference should be given to SrO, BaO or a mixture thereof, which in some compositions result in a very fine crystal structure.

In accordance with a further embodiment of the invention the bonding material also contains two oxides from the group of rare earth elements including La₂ O₃ and Y₂ O₃, one oxide of a metal having an ion radius ≦0.91 Å and one oxide of a metal having a ion radius ≧0.92 Å.

The following Table I shows the ion radii of the trivalent ions which are suitable for use.

    ______________________________________                                               ion     crystal         ion   crystal                                          radius  lattice         radius                                                                               lattice                                    ion   Å   oxide     ion   Å oxide                                      ______________________________________                                         Y.sup.3+                                                                             0.89    cubic     La.sup.3+                                                                            1.06  hexagonal                                  Dy.sup.3+                                                                            0.91    mono-     Ce.sup.3+                                                                            1.03  hexagonal                                                clinic/cub.                                                                              Pr.sup.3+                                                                            1.01  hexagonal                                  Ho.sup.3+                                                                            0.89    cubic     Nd.sup.3+                                                                            1.00  hexagonal                                  Er.sup.3+                                                                            0.88    cubic     Sm.sup.3+                                                                            0.96  monoclinic/cubic                           Tm.sup.3+                                                                            0.87    cubic     Eu.sup.3+                                                                            0.95  monoclinic/cubic                           Yb.sup.3+                                                                            0.86    cubic     Gd.sup.3+                                                                            0.94  monoclinic/cubic                           Lu.sup.3+                                                                            0.85    cubic     Tb.sup.3+                                                                            0.92                                             ______________________________________                                    

The combination 3Sc₂ O₃.3Ln₂ O₃.10Al₂ O₃ provides a melting temperature below 1700° C. if Ln₂ O₃ has a cation radius r_(i) ≧0.95 Å. If Sc₂ O₃ and Ln₂ O₃ are partly replaced by rare earth oxides having a relative small cation radius, for example by Y₂ O₃, or by Ti₂ O₃, then this may cause the melting point to be reduced in many cases.

In accordance with a further embodiment of a variant of the invention there are within the system Al₂ O₃ --Ti₂ O_(x) --Ln₂ O₃ two ranges of compositions of bonding materials, which because of the low melting range sinter very readily, more specifically the ranges of compositions in a molar percentage:

Ti₂ O_(x) --40-50

Ln₂ O₃ --15-25 and

Al₂ O₃ --30-40,

and

Ti₂ O_(x) --15-25

Ln₂ O₃ --10-20 and

Al₂ O₃ --60-70

Ln represents in both cases one or more rare earth metals, namely in sequence of preference in the first case Sm>Gd>Tb>Ge>Y and in the second case Sm>Gd>Tb.

By means of these bonding materials the composite body of the invention can be produced at a comparatively low temperature.

BRIEF DESCRIPTION OF THE DRAWING

In the drawing,

FIG. 1 is a cross-sectional view of one end of a high-pressure sodium vapor discharge lamp employing the composite body construction of the invention.

FIG. 2 is a cross-sectional view of one end of another high-pressure sodium vapor discharge lamp employing the composite body construction of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Example 1

As a starting material for the bond to be made in order to obtain a composite body in accordance with the invention, a number of compositions are melted to form a vitreous compound and a vitreous compound with small crystals, respectively in a high-frequency oven, the compound is ground to powder and subsequently, after the addition of a binder, rings are pressed from the compound and sintered.

The starting material consists of an oxide mixture chosen from aluminium oxide having a purity of 99.8%, scandium oxide, gadolinium oxide, praseodymium oxide, terbium oxide, samarium oxide, yttrium oxide, lanthanum oxide, cerium oxide, neodymium oxide, also having a purity of 99.8% and titanium (III)-oxide and/or titanium (IV)-oxide having a purity above 99.995%. The melting temperatures (Tm) vary from 1600°-1750° C. The following Tables show a number of compositions.

Table II shows a number of ternary compositions of Al₂ O₃, Sc₂ O₃ and a rare earth metal oxide,

Table III shows a number of quaternary compositions and Table IV shows some ternary compositions of Al₂ O₃, Gd₂ O₃ and Ti₂ O_(x), wherein x is varied from 2 to 4, and a number of ternary compositions with other rare earth metal oxides instead of Gd₂ O₃.

Table V describes some ternary and quaternary compositions with Ti₂ O₃.

Finally, Table VI describes different compositions, some of which contain a crystal growth-inhibiting admixture in accordance with a preferred embodiment.

                                      TABLE II                                     __________________________________________________________________________     composition in mole %                Tm                                        No                                                                               Al.sub.2 O.sub.3                                                                   Sc.sub.2 O.sub.3                                                                   Ce.sub.2 O.sub.3                                                                   Pr.sub.2 O.sub.3                                                                   Y.sub.2 O.sub.3                                                                   Tb.sub.2 O.sub.3                                                                   Sm.sub.2 O.sub.3                                                                   Nd.sub.2 O.sub.3                                                                   Eu.sub.2 O.sub.3                                                                   (°C.)                              __________________________________________________________________________     1 55  27  18                         1640                                      2 65  17.5    17.5                   1655                                      3 65  28          7                  1745                                      4 65  10.5           24.5            1735                                      5 65  21                 14          1610                                      6 70  15                     15      1700                                      7 65  17.5                   17.5    1640                                      8 70  15  15                         1635                                      9 65  17.5                                                                               17.5                       1610                                      10                                                                               60  24  16                         1635                                      11                                                                               65  17.5               17.5        1600                                      12                                                                               70  15                 15          1620                                      13                                                                               70  9                          21  1660                                      __________________________________________________________________________

                  TABLE III                                                        ______________________________________                                         composition in mole %                                                          No   Al.sub.2 O.sub.3                                                                       Sc.sub.2 O.sub.3                                                                       Y.sub.2 O.sub.3                                                                     Nd.sub.2 O.sub.3                                                                     La.sub.2 O.sub.3                                                                     Ce.sub.2 O.sub.3                                                                     Tm (°C.)                    ______________________________________                                         14   60      14      10   16                1620                               15   60      22      6          12          1623                               16   60      18      6          16          1630                               17   65      12.25   5.25       17.5        1635                               18   65      15.75   5.25             14    1615                               19   65      8.75    5.25 21                1615                               20   65      19.25   5.25 10.5              1645                               21   70      7.5     4.5  18                1635                               22   70      10.5    7.5  12                1610                               23   55      24.75   6.75             13.50 1630                               24   55      20.25   6.75             18    1630                               25   55      15.75   11.25                                                                               18                1660                               26   65      19.25   5.25       10.5        1620                               27   66      8.5     8.5  17                1595                               28   70      4.5     10.5       15          1660                               ______________________________________                                    

                                      TABLE IV                                     __________________________________________________________________________              composition in mole %      Ts                                               No.                                                                               Al.sub.2 O.sub.3                                                                    Ti.sub.2 O.sub.x                                                                   La.sub.2 O.sub.3                                                                    Ce.sub.2 O.sub.3                                                                   Sm.sub.2 O.sub.3                                                                    Gd.sub.2 O.sub.3                                                                   (°C.)                               __________________________________________________________________________     Ti.sub.2 O.sub.3                                                                     29 41.7 33.3              25.0                                                                               1700                                             30 42.0 40.0              18.0                                                                               1635                                             31 37.5 40.0              22.5                                                                               1720                                             32 40.0 42.0              18.0                                                                               1595                                             33 50.0 40.0              10.0                                                                               1615                                             34 10.0 18.0                                                                               72.0              1690                                             35 40.0 42.0                                                                               18.0              1615                                             36 10.0 27.0     63.0         1685                                             37 30.0 49.0     21.0         1625                                             38 40.0 42.0     18.0         1590                                             39 10.0 63.0         27.0     1640                                             40 20.0 56.0         24.0     1575                                             41 30.0 49.0         21.0     1565                                             42 40.0 42.0         18.0     1560                                             43 50.0 35.0         15.0     1590                                             44 60.0 20.0         20.0     1605                                             45 70.0 21.0         9.0      1610                                             46 30.0 49.0              21.0                                                                               1585                                             47 40.0 42.0              18.0                                                                               1595                                       Ti.sub.2 O.sub.3.33                                                                  48 46.7 33.3              20.0                                                                               1620                                             49 41.7 33.3              25.0                                                                               1670                                             50 41.25                                                                               33.75             25.0                                                                               1600                                             51 36.7 33.3              30.0                                                                               1670                                             52 45.05                                                                               27.65             27.3                                                                               1600                                       Ti.sub.2 O.sub.3,5                                                                   53 42.0 40.0              18.0                                                                               1615                                       Ti.sub.2 O.sub.4                                                                     54 46.7 33.3              20.0                                                                               1590                                       __________________________________________________________________________              Composition in mole %      Tm                                         No.      Al.sub.2 O.sub.3                                                                   Ti.sub.2 O.sub.3                                                                   Gd.sub.2 O.sub.3                                                                   Yb.sub.2 O.sub.3                                                                   Y.sub.2 O.sub.3                                                                   Sc.sub.2 O.sub.3                                                                   Ce.sub.2 O.sub.3                                                                   (°C.)                               __________________________________________________________________________     55       41.25                                                                              33.75                                                                              18.75                                                                              6.25           1665                                       56       41.25                                                                              33.75                                                                              12.50                                                                              12.50          1690                                       57       41.25                                                                              33.75                                                                              6.25                                                                               18.75          1710                                       58       65.5                                                                               17.5        17.0       1745                                       59       60.5                                                                               24.0        15.5       1735                                       60       56.0                                                                               29.5        14.5       1710                                       61       52.0                                                                               34.5        13.5       1715                                       62       65.0                                                                               5.25           22.75                                                                              7.0 1665                                       63       65.0                                                                               5.25           19.75                                                                              10.5                                                                               1630                                       64       65.0                                                                               8.75           15.75                                                                              10.5                                                                               1645                                       65       65.0                                                                               12.25          12.25                                                                              10.5                                                                               1630                                       66       65.0                                                                               12.25          8.75                                                                               14.0                                                                               1625                                       67       65.0                                                                               12.25          5.25                                                                               17.5                                                                               1655                                       __________________________________________________________________________

                                      TABLE VI                                     __________________________________________________________________________     composition in mole %                                                                                                crystal                                                                        growth                                   No.                                                                               Al.sub.2 O.sub.3                                                                   Sc.sub.2 O.sub.3                                                                   Y.sub.2 O.sub.3                                                                   La.sub.2 O.sub.3                                                                   Nd.sub.2 O.sub.3                                                                   Ce.sub.2 O.sub.3                                                                   Gd.sub.2 O.sub.3                                                                   Ti.sub.2 O.sub.x                                                                       inhibitor                                                                             Tm (°C.)                   __________________________________________________________________________     68 63.7                                                                               18.86                                                                              5.15                                                                              10.29                   MgO 2.0                                                                               1630                              69 62.5                                                                               8.15                                                                               4.10                                                                              8.56                    MgO 16.69                                                                             1735                              70 63.25                                                                              17.0                                                                               4.50                                                                              9.25                    SrO 6.0                                                                               1620                              71 63.25                                                                              7.75                                                                               7.75   15.25               SrO 6.0                                                                               1680                              72 59.40                                                                              15.90                                                                              4.10                                                                              8.60                    SrO 12.0                                                                              1605                              73 59.50                                                                              7.15                                                                               7.15   14.20               SrO 12.0                                                                              1700                              74 59.40                                                                              15.90                                                                              4.10                                                                              8.60                    CaO 12.0                                                                              1535                              75 59.50                                                                              7.15                                                                               7.15   14.20               CaO 12.0                                                                              1665                              76 59.40                                                                              15.90                                                                              4.10                                                                              8.60                    BaO 12.0                                                                              1585                              77 59.50                                                                              7.15                                                                               7.15   14.20               BaO 12.0                                                                              1700                              78 64.7                                                                               8.33                                                                               8.33   16.66               MgO 2.0                                                                               1610                              79 63.7                                                                               17.15          17.15           MgO 2.0                                                                               1620                              80 40.23                  24.38                                                                              Ti.sub.2 O.sub.3                                                                    32.9                                                                              P.sub.2 O.sub.5                                                                    2.49                                                                              1615                              81 40.21                  24.37                                                                              Ti.sub.2 O.sub.3.33                                                                 32.9                                                                              P.sub.2 O.sub.5                                                                    2.52                                                                              1600                              82 59.4                                                                               15.9                                                                               4.1                                                                               8.60                    ZrO.sub.2                                                                          12.0                                                                              1615                              83 59.5                                                                               7.15                                                                               7.15   14.2                ZrO.sub.2                                                                          12.0                                                                              1700                              84 61.75                                                                              18.29                                                                              4.99                                                                              9.98                    SiO.sub.2                                                                          5.0                                                                               1565                              85 55.25                                                                              16.36                                                                              4.46                                                                              8.92                    SiO.sub.2                                                                          15.0                                                                              1528                              86 52.0                                                                               15.4                                                                               4.2                                                                               8.40                    SiO.sub.2                                                                          20.0                                                                              1495                              87 62.7                                                                               8.08                                                                               8.08   16.15               SiO.sub.2                                                                          5.0                                                                               1640                              88 56.1                                                                               7.22                                                                               7.22   14.45               SiO.sub.2                                                                          15.0                                                                              1573                              89 52.8                                                                               6.8 6.8    13.6                SiO.sub.2                                                                          20.0                                                                              1550                              90 39.2    10.8               Ti.sub.2 O.sub.3                                                                    50.0      1670                              91 56.0    14.5               Ti.sub.2 O.sub.3                                                                    29.5      1710                              92 65.0                                                                               9.15                                                                               2.60                                                                              5.25                    SrO 18.0                                                                              1660                              93 65.0                                                                               4.25                                                                               4.25   8.50                SrO 18.0                                                                              1630                              94 65.0                                                                               9.15                                                                               2.60                                                                              5.25            BaO  18.0                                                                              1660                                     95 65.0                                                                               4.25                                                                               4.25   8.50                BaO 18.0                                                                              1650                              __________________________________________________________________________

By means of the above-mentioned bonding materials bonds are made between molybdenum and density sintered aluminum oxide (dga), niobium and dga, tantalum and dga and two dga parts together, and bonds of peroskites of the type ABO₃, garnets of the type A₃ Al₅ O₁₂, sapphire, together and to the said metals.

Example 2

A discharge vessel for a high-pressure sodium lamp with mercury vapour addition has at both ends a construction as shown in FIG. 1 or FIG. 2. The lamp which is known per se from inter alia U.S. Pat. No. 3,716,743 will not be further described here.

In FIG. 1, the discharge vessel consists of a tube 1 of dga and a ring 2, also of dga. A niobium tube 3 is sealed in a vacuum-tight manner in the ring. The tungsten filament 5, which serves as an electrode is sealed in the tube 3 by means of titanium (4). The tube 3 and the ring 2 are bonded together in a vacuum-tight manner by means of bonding material 6 having one of the compositions 26, 68, 69, 70, 27, 76, 72, 92, 93, 94, 95, 78, 9, 79, 42, 44, 45, 80, 81, 90 and 91 of the preceding Tables.

The construction shown in FIG. 2 comprises a tube 11, a plug 13 and a ring 12, all made of dga. A solid niobium pin 14 is passed through the plug 13. The spiralized tungsten electrode 15 is attached to the end of said pin 14. By means of a bonding material 16 having one of the above-mentioned compositions, the parts are bonded together in a vacuum-tight manner, starting from pressed rings.

The sealing process is performed as follows. First the parts to be bonded are heated, with sealing material provided between them, from room temperature in 15 sec. to 1250° C. in an oven, kept at this temperature for 60 sec., thereafter heated in 30 sec. to a temperature Ts which is 50° C. above the melting temperature (Tm) and kept at this temperature for 20 sec. Thereafter the assembly is allowed to cool in the oven to 1250° C. which temperature is maintained for 120 sec., allowed to cool thereafter in the oven to 1125° C. which temperature is maintained for 60 sec. whereafter the assembly is allowed to cool to room temperature in the switched-off oven.

Lamps comprising discharge vessels thus produced and having the customary filling of sodium vapour or metal halide and mercury vapour as the case may be were life-tested by igniting them and keeping them at the operating voltage. After 100 hours the luminous intensity of none of the lamps had decreased. The best results were obtained with lamps with a bonding material in accordance with the compositions nos. 76, 72, 92, 93, 94 and 95. 

What is claimed is:
 1. A composite body formed from two or more parts, at least one of which consists of an oxidic material, which which oxidic material is bonded to one or more parts of e metallic material or a mixture thereof with a metal oxide (cermet), the oxidic material being chosen from densely sintered aluminum oxide (dga), rare earth perowskites of the types ABO₃ wherein A represents La, Ce, Pr, Nd, or a mixture thereof and B represents: Al, Sc, Lu or a mixture thereof, garnets of the type A₃ Al₅ O₁₂, wherein A is Mg, Li or Y, or sapphire, and the metallic part being chosen from tantalum, niobium, tungsten, molybdenum, alloys having one of these metals as the main component and alloys having iron, nickel or cobalt as the main component, molybdenum-silicon alloys, titanium boride and titanium nitride, which parts are bonded together in a gas and vacuum-tight manner with a bonding material, the bonding material containing aluminum oxide and at least one oxide of a rare earth metal, characterized in that the bonding material consists of a finely dispersed polycrystalline product comprising at least three oxidic components and whose composition is within the following limits in mole %Al₂ O₃ --5-70at least one oxide chosen from Sc₂ O₃ and Ti₂ O_(x), wherein 2<x<4--2-70at least one oxide chosen from Y₂ O₃, Gd₂ O₃, Tb₂ O₃, Dy₂ O₃, Ho₂ O₃, Er₂ O₃, Eu₂ O₃, Pr₂ O₃, Ce₂ O₃, Sm₂ O₃, Tm₂ O₃, Yb₂ O₃, Lu₂ O₃, La₂ O₃ and Nd₂ O₃ --7-80and one or more oxides chosen from ZrO₂, B₂ O₃, SiO₂, P₂ O₅, MgO, SrO, BaO and CaO--0-20.
 2. A composite body as claimed in claim 1, characterized in that the polycrystalline product comprises SrO, BaO or a mixture thereof.
 3. A composite body as claimed in claim 1 or 2, characterized in that the bonding material contains two oxides from the group of rare earth metals including La₂ O₃ and Y₂ O₃, one oxide of a metal having ion radius ≦0.91 Å and one oxide of a metal having ion radius ≧0.92 Å.
 4. A composite body as claimed in claim 1 or 2, characterized in that the bonding material corresponds to a molar ratio of 3Sc₂ O₃.3Ln₂ O₃.10Al₂ O₃, wherein Ln is at least partly a rare earth element having ion radius ≧0.95 Å.
 5. A composite body as claimed in claim 1, characterized in that the bonding material has a composition within the following range of composition in mole %Ti₂ O_(x) --40-50 Ln₂ O₃ --15-25 and Al₂ O₃ --30-40wherein Ln represents one or more rare earth metals in the sequence of preference Sm>Gd>Tb>Ce>Y.
 6. A composite body as claimed in claim 1, characterized in that the bonding material has a composition within the following range of composition in mole %Ti₂ O_(x) --15-25 Ln₂ O₃ --10-20 and Al₂ O₃ --60-70wherein Ln represents one or more rare earth metals in the sequence of preference Sm>Gd>Tb.
 7. A gas discharge lamp having an envelope of densely sintered aluminum oxide, an electrode feedthrough member of niobium, a gas filling consisting during operation of sodium vapour, the aluminum oxide envelope parts being bonded together and to the niobium feed-through member by means of bonding material so as to constitute a composite body as claimed in claim
 1. 